System for management of mechanical stress in nitinol components

ABSTRACT

A self-limiting shaped memory alloy device, including a shape memory alloy member, with first and second ends, a first anchor member connected to the first end, an energy contact, a second anchor member connected to the energy contact, an energy source connected in energetic communication to the energy contact, a moveable member connected to the second end, and a biasing member operationally connected to the moveable member for urging the moveable member towards and into physical contact with the second anchor member. The moveable member is in physical contact with the second anchor member and the second end is in energetic communication with the energy contact. Actuation of the energy source energizes the energy contact. Energization of the shape memory alloy member initiates a phase change that urges the moveable member away from the second anchor member. Movement of the moveable member away from the second anchor member disengages the second end from the energy contact.

CROSS-REFERENCE TO RELATED APPLICATION

The present patent application is a divisional of, and claims priorityto, U.S. patent application Ser. No. 14/602,731, filed on Jan. 22, 2015,now U.S. Pat. No. 9,714,460, which claimed priority to then co-pendingU.S. provisional patent application Ser. No. 61/939,436, file on Feb.13, 2014, the entirety of which is incorporated herein by reference.

TECHNICAL FIELD

This novel technology relates generally to the field of materialsscience and, more particularly, to a system for controlling and limitingthe applied energy and resultant stress in a cyclical-memory orshape-memory material component, such as a Nitinol material component,to moderate and reduce cyclic fatigue.

BACKGROUND

Shape-memory alloys (SMAs) are materials that, once deformed, return totheir original shape upon heating. SMAs are typically either of thecopper-aluminum-nickel system or nickel-titanium system of alloys,although other alloys may also exhibit SMA properties. Although not thecheapest SMAS, nickel-titanium alloys, such as Nitinol, are popular dueto their stability and superior cycling properties.

SMAs function by shifting back and forth between can two differentphases, with three different crystal structures (i.e. twinnedmartensite, detwinned martensite and austenite) and six possibletransformations. Nitinol, for example, changes from austenite tomartensite when cooled, with a specific transition temperature tomartensite upon cooling and specific temperatures upon which thetransition to austenite begins and finishes upon heating. Repeatedcycling of SMAS eventually leads material fatigue, as evidenced by acreep or drift in the specific transition temperatures. Further, ifheated beyond a maximum threshold temperature, SMAS lose their abilityto be cycled between shapes and thus are susceptible to permanentdeformation. The phase transition from the martensite to austenite isthus a function of temperature and induced stress, but not a function oftime.

SMAs may remember only a ‘cold’ shape, to which a deformed SMA returnsupon heating and then cooling, or they may remember both a ‘hot’ shapeand a ‘cold’ shape, between which they may be thermally cycled. Ineither case, accumulated cycling will eventually result in fatigue. SMAsdo not have infinite cycling capacity and thus potentially long servicelifetimes limited by the number of cycles experienced and the degree ofstress experienced during each cycling event.

Currently, stress in SMAs is managed with electronic sensors orswitches. Nitinol, for example, transforms its crystal structure betweenmartensitic and austenite, with the transformation based on the energystate of the material. Temperature is a common way to measure the energystate of the material. Many devices limit the energy of their SMAcomponents by monitoring the temperature of the SMA element andregulating energy input based on the measured temperature. Alternately,some systems regulate energy input based on percent deformation of theSMA element, such as by using complex algorithms based on experimentaldata. Both of these methods require some sort of intelligence to manage.Excess energy in the SMA element will degrade the reversibletransformation life cycle. As both of the above-described methodsmeasure temperature and/or deformation as a basis for regulation, theyboth suffer from the drawback that excessive temperature and/ordeformation must be measured before regulation is initiated, but iftemperature increases or excessive deformations occur rapidly, the SMAelement incurs life-cycle shortening damage before regulation iseffectively implemented. Thus, there remains a need for a system ofregulating SMA element cycling that prevents excessive heat and/ordeformation from occurring. The present novel technology addresses thisneed.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a first perspective view of a first embodiment mechanicalstress management system of the present novel technology.

FIG. 2 is a side elevation view of FIG. 1.

FIG. 3 is a cutaway view of FIG. 2.

FIG. 4 is an exploded view of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of thenovel technology, reference will now be made to the embodimentsillustrated in the drawings and specific language will be used todescribe the same. It will nevertheless be understood that no limitationof the scope of the novel technology is thereby intended, suchalterations and further modifications in the illustrated device, andsuch further applications of the principles of the novel technology asillustrated therein being contemplated as would normally occur to oneskilled in the art to which the novel technology relates.

According to a first embodiment of the present novel technology, asillustrated in FIGS. 1-4, the present novel technology relates to asystem 10 for managing the energy and/or stress of a shape-metal alloyelement to reduce or minimize the accumulation of fatigue, thusextending the life of the SMA element. One such embodiment system 10includes an SMA element 15 (in this case, a double length of Nitinolwire including first parallel wire 17 and second parallel wire 19,although any SMA element form and composition may be elected)mechanically connected between a structural support base member or block20 and a second, typically partitioned, support member or block 25, andalso connected in energetic communication with an energy source 3 o. Inthis embodiment, the energy source 30 is an electric power supplyconnected in electric communication with the SMA element 15; however, inother embodiments the energy source may be a heat source, a lightsource, or the like.

The partitioned block 25 further includes a proximal portion 35 and adistal 4 0 portion normally abutting one another in direct physicalcontact, with an elongated member 45 extending through both portions 35,40 and clamped or otherwise mechanically affixed to both portions 35,40. The elongated member 45 is likewise operationally connected to areturn spring 50 at one end and a biasing member or overstress spring 55at the other. The return spring 50 is mechanically engaged with the baseblock 20, and the overstress spring 55 is likewise mechanically engagedwith the proximal portion 35.

Optionally, an energy management device 60 is operationally connectedbetween the energy source 30 (in this embodiment, a DC power supply) andthe SMA element 15 to control the electrical power into the SMA wireelement 15 (which in the prior art is based on wire temperature orpercent contraction of the wire). The return spring 50 provides anurging force through the elongated member 45 and the blocks 20, 25 ontothe SMA element 15 to extend the SMA wires 15 when there is no powerflowing, such as after power has been removed.

Wire 17 includes a first electrical contact 70A at one end and a secondelectrical contact 70B at the opposite end. Likewise, wire 19 includes afirst energy conducting (typically electrical) contact 75A at one endand a second energy conducting or electrical contact 75B at the oppositeend. An energy conducting contact 80, typically an electrode, electricpower clip, thermal conductor, or the like, is mechanically connected toproximal portion 35 and is engaged in electric communication withcontact 70B. Energy conducting contact 85 is physically spaced fromelectric contact 80, is mechanically connected to proximal portion 35and is engaged in electric communication with contact 75 B. Energyconducting contact 90 is mechanically engaged to distal portion 40 andwhen distal portion 40 abuts proximal portion 35, contact 90 is inmechanical and electrical connection with contacts 80 and 85.

The present novel technology further includes a biasing member such asan over stress spring or tension device 55 operationally connected tothe SMA wire element 15, in this embodiment through the elongated member45 and the blocks 20, 25, and engaged to the proximal portion 35 tolimit the maximum stress experienced by the wire 15. The over stressspring 55 provides a constant, predetermined urging force 105 on theproximal portion 35 in the direction of the distal portion 40. Theurging force 105 keeps the proximal and distal portions 35, 40 incontact while the SMA wire 15 contracts due to the phase change inducedby the heating of the wire 15 when the power source 30 is engaged. Whenthe contraction of the SMA wire element 15 generates a sufficientlygreat urging force 110 opposite that of the over stress spring 55, theproximal and distal portions 35, 40 physically move apart and separatefrom one another 35, 40, thus breaking the electrical connection betweencontact 90 and contacts 80, 85, thus ceasing the flow of energy into thewire element 15. As the wire 15 cools and extends, the urging force 110pulling the blocks 20, 25 together diminishes and the urging force 105from the overstress spring 55 once again dominates, pushing the proximalportion 35 away from the base block 20 and towards the distal portion 40until the contacts 90 and 80, 85 once again touch in electriccommunication. The returning electrical connection enables energy toflow into the wire 15, repeating the cycle until power is once againautomatically removed. The over stress spring 55 is typically sized tolimit the stress in the wire element 15 to a predetermined maximumstress. The maximum stress in the wire 15 influences the fatigue cyclelife of the wire 15, with lower maximum allowed stress producing ahigher fatigue limit. The return spring 50 is used to extend the wire 15after the power is removed. The value of the urging force 100 or thespring constant of the return spring 50 may also be manipulated tocontribute to the maximum transition energy level allowed in the wire15.

In general, the system 10 may be described as a failsafe forautomatically preventing an SMA element 15 from experiencing excessive,degenerating stress and the subsequent damage or fatigue caused by thesame. The urging force no generated by the physical dimensionaltransition accompanying the phase transition of the anchored SMA element15 is harnessed to overcome an opposing urging force 105 (as provided bythe biasing element 55) to disengage the energy source 30 driving theSMA phase transition. The energy source 30 is connected to the SMAelement 15 at the junction of two members 35, 40, which are urged apartby the dimensional transition of the SMA element against the urgingforce 105 of the biasing element 55. The shape memory alloy member 15may expand or contract upon energization and phase transition. Theurging force 105 of the biasing element 55 is calibrated to be counteredby the urging force 110 generated by the dimensional change of the SMAelement 15 before the stress in the SMA element 15 accompanying thephase transition exceeds some predetermined maximum. The predeterminedmaximum stress value is typically selected to be lower than a thresholdvalue associated with rapid fatigue of the SMA element 15 so as toprolong the service life of the SMA element 15.

In operation, the system 10 operates to automatically limit the stresslevel of a shape memory alloy member 15 to prevent service lifeforeshortening levels of stress by first connecting the shape metalalloy member 15 to a first base structural member 20 and to a secondmoveable structural member 35, and also connecting an energy inputsource 30 to an energy conducting member 90. The energy conductingmember 90 is connected to a third structural member 40. The second andthird structural members 35, 40 may be separate or connected as a singleunit 25. The third structural member 40 is positioned in contact withthe second structural member 35, and the shape metal alloy member 15 isconnected in energetic communication with the energy conducting member90. A biasing member 55 is connected to the second structural member 35to provide a first urging force 105 thereupon, wherein the first urgingforce 105 urges the second structural member 35 towards the thirdstructural member 40. The energy conducting member 90 is energized toconduct energy into the shape memory alloy member 15, generating anopposite, greater urging force 110 in the shape memory alloy member 15urging the second structural member 35 to move away from the thirdstructural member 40. The energy conducting member 90 is disengaged fromthe shape memory alloy member 15, and no longer conducts energythereinto.

While the novel technology has been illustrated and described in detailin the drawings and foregoing description, the same is to be consideredas illustrative and not restrictive in character. It is understood thatthe embodiments have been shown and described in the foregoingspecification in satisfaction of the best mode and enablementrequirements. It is understood that one of ordinary skill in the artcould readily make a nigh-infinite number of insubstantial changes andmodifications to the above-described embodiments and that it would beimpractical to attempt to describe all such embodiment variations in thepresent specification. Accordingly, it is understood that all changesand modifications that come within the spirit of the novel technologyare desired to be protected.

I claim:
 1. A method of automatically limiting the stress level of ashape memory alloy member to prevent service life foreshortening levelsof stress, comprising: a) connecting the shape memory alloy member to afirst base structural member and to a second moveable structural member;b) connecting an energy input source to an energy conducting member; c)connecting the energy conducting member to a third structural member; d)positioning the third structural member in contact with the secondstructural member; e) connecting the shape memory alloy in energeticcommunication with the energy conducting member; f) connecting a biasingmember to the second structural member to provide a first urging forcethereupon, wherein the first urging force urges the second structuralmember towards the third structural member; g) energizing the energyconducting member to conduct energy into the shape memory alloy member;h) generating an opposite, greater second urging force in the shapememory alloy member urging the second structural member to move awayfrom the third structural member; and i) disengaging the energyconducting member to conduct energy from the shape memory alloy member.2. The method of claim 1 wherein the second structural member ispositioned between the first and third structural members.
 3. The methodof claim 1 wherein the shape memory alloy member contracts whenenergized.
 4. The method of claim 1 wherein the shape memory alloymember expands when energized.